I want to add the information about the reason for the long TTFB which I discovered a few days ago (it was a long way).

The reason - especially in my case - was the WordPress Plugin Autoptimize. This plugins generates (after flushing the Autoptimize cache) in the first request the combined and minified CSS and JavaScript files (and a lot of other stuff if you use it). At the end this tooks a significant time on the site of the server. This can also happen with other plugins like W3TotalCache if you use the minification feature for example.